Question: if you're moving when you hit the OK button does it send the same location three times over 20 minutes, or is the location updated each time it sends? I suspect it's the former, right?

It is the former. It is a triple redundant packet. All messages are identical (and contain a location from right after you activated check in mode).

I'd like it if you could elaborate on the points you believe to be 10 miles off. We have recorded literally MILLIONS of fixes with the GPS chipset in SPoT units. The worst error we've ever seen is around 100 yards, and that's under abysmal signal conditions with a 2D GPS fix.

A 10 mile error (or even a one mile error) is essentially contrary to certain laws of physics.

Since the check in message that is successfully received might be the third attempt, and that third message may be sent up to 20 minutes after the GPS fix was obtained, you would only have to be traveling 30 MPH to have moved 10 miles between the time the GPS reading was taken and the message successfully got through. Is is possible that this was the case?